Write! It's an important skill used in all sorts of ways: jobs, fun, maintaining important relationships, etc. It's also good for trying out those new words you're learning through your reading. Over the summer, I plan on writing a lot of letters to friends and family, and keeping a semi-regular journal. Below are some ideas of things to write over the summer. I encourage you to keep your writing skills fresh!
A regular journal over the summer, as little as 5 minutes a day (or more, if you want). Write about your day - what happened, your thoughts and feelings and the people in your day, etc. Or you can get a family member to ask a question, and then answer it in your journal.
An email or a letter to a family member far away. Write a letter to a teacher. Bonus points for sending an actual letter through the actual mail.
A letter to one of your address's elected officials about an issue you feel strongly about. Keep it informed and respectful. Here's a website to find your local legislator: https://app.leg.wa.gov/districtfinder/.
A movie review or a book review. Less summary, more what you think and whether you'd recommend it.
A poster/public awareness/announcement/etc about something you want to share. Put up a sign on the wall of your house about how your little brother needs to do a better job of keeping the bathroom clean.
Other ideas as posted. Let me know if you think of something.